With regard to the reduction step of obtaining metallic titanium from titanium tetrachloride as an intermediate product among the steps for producing metallic titanium from titanium ore, so-called the Kroll process is most commonly adopted industrially. The following will describe the reducing process of titanium in the Kroll process with reference to FIG. 4A. First, titanium ore is chlorinated to form titanium tetrachloride which is liquid at ordinary temperature. Then, through a liquid titanium tetrachloride supply pipe 8, titanium tetrachloride is supplied to a tightly closed reduction reaction vessel 1, i.e., onto a reactant bath liquid 2 in a reaction vessel 1. Highly pure metallic titanium is obtained by changing magnesium into fused magnesium dichloride and titanium tetrachloride into metallic magnesium in the reaction vessel through the following chemical reaction.TiCl4+Mg→Ti+MgCl2  (1)
Metallic titanium precipitates as fine particles in the reaction vessel and then the particles are sintered each other to form a porous titanium sponge mass. Moreover, since specific gravity of fused magnesium dichloride as a by-product is larger than that of fused magnesium and also fused magnesium dichloride and fused magnesium hardly dissolve in each other, magnesium dichloride precipitates on the bottom of the vessel to form a product bath liquid layer 3 and forms a definite reactant-product bath liquid interface 4 between the layer 3 and a reactant bath liquid layer 2. After precipitation, magnesium dichloride formed in the reactant bath liquid is absorbed in the product bath liquid 3. The volume of the reduction bath liquid gradually increases as a result of the formation of the product during the reduction reaction but the product bath liquid is adequately discharged to the outside of the vessel through a product bath liquid-discharge pipe 9 by forcing a bath surface 5 caused by periodical introduction of high-pressure argon into a space above the bath surface 5 through an argon gas supply pipe 10. As a result, the level 19 of the bath surface is maintained within a certain range. After the accumulated supply of titanium tetrachloride has reached a predetermined level, the reduction bath liquid is discharged to the outside of the vessel and the titanium sponge is taken out of the vessel as a product after separating the reduction bath liquid remained in the voids by heating in vacuo. In the case of a recent representative large-scale reduction reaction apparatus, the size of the reaction vessel is up to a diameter of about 2 m, a height of about 5 m, and a depth of the reduction bath liquid of 4 m, and a little less than 10 tons of titanium sponge is produced at one batch production.
The “reactant bath liquid” 2 herein means a liquid layer in the reaction vessel containing fused magnesium as a main component and titanium tetrachloride, and is present at an upper part in the bath liquid owing to its small average density. Moreover, the “product bath liquid” 3 means a liquid layer in the reaction vessel containing fused magnesium chloride as a main component and formed titanium fine particles, and is present at a lower part in the bath liquid owing to its large average density. Furthermore, the “reduction bath liquid” includes both of the reactant bath liquid and the product bath liquid. The “reactant-product bath liquid interface” means an interface between the reactant bath liquid layer and the product bath liquid layer.
The titanium sponge mass formed in the reduction step is classified into a titanium sponge large mass part 6 and a sponge upper wall part 7 and both of them grow individually. The titanium sponge large mass 6 is a large mass which grows upward from the bottom of the reaction vessel and accounts for most part of total weight of the sponge mass. Moreover, the sponge upper wall part 7 is a sponge mass which grows from the inner wall of the reaction vessel near the bath surface toward the inside of the radical direction of the reaction vessel.
In general, the larger the weight of titanium sponge producible in one batch is, the higher the productivity is and the lower the production cost is. The weight of titanium sponge producible in one batch is determined by the quantity of titanium tetrachloride supplied until the titanium sponge large mass grows and the top of the large mass reaches the bath surface of the reduction bath liquid. This is because the direct contact of titanium tetrachloride liquid supplied with the titanium sponge mass results in an unstable reduction reaction and causes problems of the clogging of the titanium tetrachloride-supply pipe and contamination of product titanium and hence reduction should be finished at the point of time when the top of the large mass reaches the bath surface of the reduction bath liquid in order to avoid the problems. In the conventional art, the titanium sponge large mass had a conical shape as shown in FIG. 4B. Therefore, there existed a large space filled with the reduction bath liquid between the titanium sponge large mass and the inner wall of the cylindrical vessel at the point of time when reduction was finished, and thus there was a problem that the production of titanium sponge per one batch decreased.
Some attempts have been hitherto made for solving the problem. For example, JP-A-8-295955 aims at increase of the average diameter of the sponge large mass to allow it to grow in a pillar form by supplying titanium tetrachloride over a wide range of the reduction bath liquid dispersively. (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.) However, although JP-A-8-295955 is silent about a size of the reaction vessel, as a result of precise investigations by the present inventors, it has been found that the region of the sponge mass whose average diameter is increased by this method is limited to the depth range shallower than 500 mm below the bath surface of the reduction bath liquid and thus it is only effective in a very small part of the sponge large mass having a height of more than 3 m in the current representative reduction reaction apparatus. Furthermore, a diameter-increasing effect on the titanium sponge large mass by this method within the depth range shallower than 500 mm below the bath surface of the reduction bath liquid is only a little and the shape of the whole titanium sponge large mass is still regarded as a conical shape. This is because a large circulating flow exists in the reduction bath liquid. That is, even when the titanium tetrachloride-supplying spots are diverged to expand the metallic titanium particle-generating spots on the bath surface, produced titanium particles are stirred by the circulating flow in the most part of the bath liquid, so that the difference in the effect from the case that titanium tetrachloride is supplied only to the central part of the bath surface becomes small.
The reason why the conical shape titanium sponge large mass is formed irrespective of the titanium tetrachloride-supplied position in the case that the circulating flow exists in the reduction bath liquid has been hitherto unknown. As a result of precise investigations by the inventors, however, it has been found for the first time that this phenomenon is due to a positional change of the reactant-product bath liquid interface. The following will describe the specific mechanism.
Although a level 20 of the reactant-product bath liquid interface may fluctuate during the reducing reaction, the level of the reactant-product bath liquid interface tends to increase as the passage of the reaction time when the overall reduction reaction is considered from a broader perspective. An average increase of the level of the reactant-product bath liquid interface per unit weight of titanium tetrachloride supply is defined as an “elevating rate of the reactant-product bath liquid interface” θ. As a result of the precise investigations, the inventors have found that a relationship shown in FIG. 5 is present between the elevating rate of the reactant-product bath liquid interface θ and the diameter of the titanium sponge large mass. The “average diameter of the titanium sponge large mass” means an average sponge diameter in the vertical direction of the large mass when the titanium sponge large mass is regarded as a cone or a cylinder or a shape composed of a cylinder overlaid with a cone. In FIG. 5, the tendency of the relationship between θ and the average diameter of the titanium sponge large mass changes at a point (a) as a border. That is, when θ is larger than the point (a), the average diameter of the titanium sponge large mass increases as θ decreases. This is because a height 21 of the titanium sponge large mass is regulated by the level 20 of the reactant-product bath liquid interface and cannot exceed the level 20 of the interface to a large extent. As a result, when θ is large, the titanium sponge large mass can grow upward and hence the average diameter of the titanium sponge large mass decreases. To the contrary, when θ is small, the upward growth of the titanium sponge large mass is suppresed and the mass mainly grows in the radial direction, so that the average diameter of the titanium sponge large mass increases. On the other hand, when θ is smaller than the point (a), the average diameter of the titanium sponge large mass has a constant value irrespective of θ. This is because the titanium sponge large mass can no longer grow in the radial direction in this region since the sponge has grown in the radial direction to come into contact with the inner wall of the reaction vessel. In the conventional art, the operation point of θ is determined so as to maintain a level 19 of the bath surface constant during the reduction reaction. When each physical value is substituted for the chemical equation of the formula (1), the volume of the reduction bath liquid increases by about 0.5 m3 by the formation of a product in the case that 1 t of titanium tetrachloride is supplied into the reaction vessel. In the conventional art, in order to maintain the level of the bath surface constant, i.e., to maintain the volume of the reduction bath liquid constant, the volume of the reduction bath liquid increased by the reaction should be discharged to the outside of the reaction vessel. Because the reduction bath liquid to be discharged is a product bath liquid containing magnesium dichloride as a main component, it is enough to discharge about 0.82 t of the product bath liquid per 1 t of the titanium tetrachloride supply, i.e., to set a discharge rate of the product bath liquid at 0.82 t (product)/t (titanium tetrachloride) based on the calculation using the physical values of magnesium dichloride. Since the discharge rate of the product bath liquid corresponds to θ in one-to-one manner, θ in the conventional art becomes a fixed condition and it is found that the conventional operation condition exists in a region where θ is larger than the point (a) as shown in FIG. 5. Therefore, in the conventional art, the average diameter of the titanium sponge large mass is smaller than the average inner diameter of the reaction vessel to a large extent and hence a titanium sponge large mass having a large weight through full utilization of the space in the reduction bath liquid cannot be formed.
The following will describe the reason why the level 20 of the reactant-product bath liquid interface determines the height 21 of the titanium sponge large mass. This phenomenon has also been found for the first time based on the results of precise investigations by the inventors. First, precipitation behavior of formed metallic titanium particles 18 in the reduction bath liquid will be explained with reference to FIG. 9. Titanium tetrachloride is supplied from above the bath surface and hence metallic titanium particles are formed near the bath surface (point (a)). Since the density of the metallic titanium particles is larger than the average density of the reduction bath liquid, the metallic titanium particles precipitates and settles on the titanium sponge large mass to allow to grow the titanium sponge large mass. Three kinds of the precipitation and settling routes of the metallic titanium are present when roughly classified. The first route is a route (b) wherein the metallic titanium particles pass through the reactant-product bath liquid interface and attach to the sponge mass in the product bath liquid. The second route is a route (c) wherein the metallic titanium particles are transported by the circulating flow present in the reactant bath liquid to descend along the inner wall of the reaction vessel and without passing through the reactant-product bath liquid interface, transferred in the central direction of the vessel to attach to the skirt of the sponge mass exposed above the interface. The third route is a route (d) wherein the metallic titanium particles descend and directly attach to the titanium sponge large mass exposed to the reactant bath liquid without coming into contact with the reactant-product bath liquid interface. Of these three routes, the route (c) is always a main route. The reasons are as follows. First, the reason why the route (b) hardly occurs is that the size of the metallic titanium particles is usually extremely small, e.g., about several tens μm or less, and hence the particles cannot easily pass through the reactant-product bath liquid interface. This is because, when the particles break through the interface, gravitational force should overcome a resisting force against particle precipitation owing to the curved interface, i.e., a resisting force according to the Laplace equation, and a resisting force against particle precipitation derived from interfacial tension imparted at the time when metallic titanium particles existing in the wetting reactant bath liquid intrude into the less wetting product bath liquid layer. The Laplace equation is expressed by the following equation. At the precipitation of the particles, a static pressure between the particles and the reactant-product bath liquid interface is elevated by deforming the reactant-product bath liquid interface into a downward convex shape, and thereby the particle precipitation is resisted.                                               ⁢                              [                          Static              ⁢                                                          ⁢              pressure              ⁢                                                          ⁢              near              ⁢                                                          ⁢              two              ⁢                                                          ⁢              liquid              ⁢                                                          ⁢              interface                        ]                    =                      2            ×                                          [                                  Interfacial                  ⁢                                                                          ⁢                  tension                  ⁢                                                                          ⁢                  between                  ⁢                                                                          ⁢                  two                  ⁢                                                                          ⁢                  liquid                                ]                            /                                                                 [                                      Curvature                    ⁢                                                                                  ⁢                    radius                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    interface                    ⁢                                                                                  ⁢                    between                    ⁢                                                                                  ⁢                    two                    ⁢                                                                                  ⁢                    liquid                                    ]                                                                                        (        2        )            
In the case of fine particles having a larger surface area relative to the volume, gravitational force seldom exceeds such a resisting force imparted to the surface. As a result of the investigations by the inventors, it has been found that the particle size of metallic titanium should be at least about several mm for realizing the passage through reactant-product bath liquid interface. Since such large particles exist in only a small amount in the bath, a small ratio of metallic titanium particles passes through the route (b). The following will explain the reason why the route (d) hardly occurs. The ratio of metallic titanium particles passing along the route (d) increases as the height of the titanium sponge large mass increases and the metallic titanium-forming position comes near the top of the large mass. However, in the case that the top of the large mass is, e.g., 500 mm or more apart from the bath surface, most of the formed metallic titanium particles are once transported to the outside of the radial direction by the circulating flow existing under the bath surface, and then transferred to the reactant-product bath liquid interface by the circulating flow along the inner wall of the reaction vessel, so that the ratio of the particles passing along the route (d) is small. Therefore, the route (c) is a main route for precipitation of metallic titanium particles. In the route (c) as a main route, since titanium sponge mass grows by attaching most of the formed metallic titanium to the titanium sponge large mass in the reactant bath liquid just above the reactant-product bath liquid interface, it is not probable that the titanium sponge large mass rapidly grow during the reduction reaction in the height region where the reactant-product bath liquid interface does not yet reach. In this sense, it can be said that the level of the reactant-product bath liquid interface determines the height of the titanium sponge large mass.
The following will describe the reason why the titanium sponge large mass forms a conical shape in the conventional art. In the conventional art, since the elevation rate of the reactant-product bath liquid interface is large, the titanium sponge large mass does not grow largely in the radial direction except the bottom part of the titanium sponge large mass and grows upward in a long and narrow form. At that time, since the titanium sponge is formed at an early stage of the reduction reaction at the lower part of the large mass, the titanium sponge grows over a longer period of time. The route (b) in FIG. 9 is not a main route for precipitation and attachment but there exists in a certain ratio, so that metallic titanium particles attaches to the side surface of the titanium sponge mass at the lower part where the titanium sponge grows over a long period of time and hence the diameter of the titanium sponge mass increases at the lower part. On the other hand, such a diameter-increasing mechanism of the titanium sponge mass is difficult to work because only a short period of time has passed from the beginning of the formation of the sponge at the upper part of the titanium sponge mass and the rate of the attachment of titanium particles by the route (d) shown in FIG. 9 is larger at the higher position of the titanium sponge large mass, so that the titanium sponge large mass shows a tendency of growing selectively upward. As a result, the sponge mass grows conically in the order of (a)→(b)→(c) shown in FIG. 10.